1. Field of the Invention
One disclosed aspect of the embodiments relates to an image forming apparatus having a fixing unit for heat fixing an unfixed toner image formed on a recording sheet onto the recording sheet.
2. Description of the Related Art
Image forming apparatuses such as copying machines and laser beam printers are provided with fixing units. Such fixing units can be classified into several types, for example, a heat roller type fixing unit having a halogen heater as a heat source, and a film heating fixing unit having a ceramic heater as a heat source.
The heater provided in the fixing unit is connected to a commercial alternating-current (AC) power supply via a switching element such as a triac. From the commercial AC power supply, electric power is supplied to the heater. The fixing unit is provided with a temperature detection element for detecting temperature in the fixing unit, for example, a thermistor. A central processing unit (CPU) performs on/off control of the switching element such that a temperature detected with the temperature detection element is kept to a target temperature. Such power control keeps the temperature of the fixing unit to the target temperature. The on/off control to the heater is performed in phase control or wave number control.
In the phase control, the electric power to be supplied to the heater is controlled by turning on the switching element at an arbitrary phase angle within half-cycle of an AC waveform. In the wave number control, the electric power to be supplied to the heater is controlled by turning on the switching element in units of half-cycle of the AC waveform.
The phase control is employed to suppress flickering of a lighting apparatus, the phenomenon is called flicker. The flicker refers to the flickering of the lighting apparatus that occurs when the AC power supply produces voltage fluctuations due to load current fluctuations of an electrical apparatus connected to the same power supply as the lighting apparatus and an impedance of a distribution line. In the phase control, the electric current flows at each half-cycle, and the amount of change and the period of change of the current are small, which suppresses the occurrence of the flicker. Meanwhile, in the wave number control, the switching element is turned on and off in half-cycle units of the commercial AC power supply, and this generates more current fluctuations than those in the phase control. Consequently, in the wave number control, the flicker is more likely to occur.
The wave number control is employed to reduce harmonic current and switching noise. The harmonic current and switching noise are produced due to rapid current change in the turning on/off operation of the heater. In the wave number control, the on/off operation of the heater is always performed at zero-cross points, and consequently, the harmonic current and switching noise are less likely to occur as compared to the phase control in which the switching operation is performed in the middle of the half-cycles of the AC waveforms. The harmonic current and switching noise tend to occur to a larger extent with a higher voltage of the AC power supply being used.
In view of the above, it is general to fix a power control method depending on the commercial AC power supply voltage in the region the image forming apparatus is used. For example, apparatuses designed for the regions of the commercial AC power supply voltage of 100 to 120 V employ the phase control method which is advantageous to the flicker, and apparatuses designed for the regions of the commercial AC power supply voltage of 220 to 240 V employ the wave number control method which is advantageous to the harmonic current and switching noise.
In addition to the control methods, methods of combining the phase control and the wave number control are discussed. For example, Japanese Patent Application Laid-Open No. 2011-18027 discusses a method in which a part of half-cycles in one control cycle consisting of a plurality of half-cycles is controlled in the phase control, and the rest of the half-cycles are controlled in the wave number control. According to the method, as compared to the case the power supply is performed only in the phase control, the occurrence of the harmonic current and switching noise can be suppressed. Further, the method enables reduction of the flicker as compared to the case the power supply is performed only in the wave number control, and the power control to the heater can be controlled by further multilevel control.
In the description, a positive half-cycle for supplying electric power in the phase control or the wave number control is defined as a positive energization cycle, and similarly a negative half-cycle for supplying electric power is defined as a negative energization cycle. Further, a half-cycle that is not supplying electric power is defined as a non-energization cycle. One unit period for controlling the amount of power to be supplied to the heater by dividing the period into certain periods is defined as one control cycle. In the description below, as an example, a method of updating power supply to a heater and an upper limit current value in one control cycle consisting of four full-cycles (eight half-cycles) is described.
To perform power control of a fixing unit, a sequence controller compares a temperature detected with a temperature detection element to a preliminary set target temperature, and calculates a power ratio (power level) to be supplied to the heater. Then, the sequence controller determines a phase angle or a wave number corresponding to the power ratio, and under the phase condition or the wave number condition, a switching element for driving the heater is turned on or off.
There is a general tendency that the unevenness of heat generation in one control cycle can be reduced by increasing the number of times of phase control in the one control cycle. The reduction of the unevenness of heat generation of the heater for balancing the heat quantity to be applied to the recording sheet enables increase in the print quality and fixability. As described above, the wave number control is advantageous to the harmonics. Consequently, in actual control operation, it is desirable to employ a control pattern in which the number of times of the wave number control is increased as much as possible in a range satisfying the harmonic specifications.
Meanwhile, some heaters have characteristics that a resistance value varies with change in temperature. The degree of change is represented by a temperature coefficient of resistance. If the resistance value increases in proportion to temperature increase, then, the temperature coefficient is referred to as a positive temperature coefficient (PTC) (positive temperature characteristics of resistance), and if the resistance value decreases in inverse proportion to temperature increase, then, the temperature coefficient is referred to as a negative temperature coefficient (NTC) (negative temperature characteristics of resistance).
Influence of the temperature coefficients of resistance of the heater will be described. Image forming apparatuses are provided with various print modes to handle various types of paper to be used, differences in usage environments, and the like. Depending on the type of paper and the usage environment, optimal fixing conditions vary, and consequently, the target temperature is changed under the individual conditions. To change the target temperature means to change the heat generation temperature of the heater, and if the temperature is changed, due to the influence of the temperature coefficients of resistance, the resistance value of the heater fluctuates. As a result, the electric current flowing through the heater also fluctuates. As described above, there is a close connection between the heater current and the harmonics, and the current fluctuations affect the harmonic level.
First, a PTC heater having positive resistance temperature characteristic is described. The heater resistance value increases in proportion to temperature increase. In a state the target temperature is set to a high temperature is used as a reference, within a range the harmonic specifications can be satisfied, if generation of a control pattern in which the number of times of the phase control in one control cycle is increased as much as possible is performed, when the target temperature is lowered, due to the increasing heater current, the harmonic specifications are not satisfied. On the other hand, in a state the target temperature is low is used as a reference, if generation of a control pattern in which the number of times of the phase control is small is performed, when the target temperature is increased, the harmonic specifications are satisfied. However, the heat generation unevenness of the heater is large, and this is disadvantageous to image quality of print image.
Next, an NTC heater having negative resistance temperature characteristic is described. The heater resistance value decreases in inverse proportion to temperature increase. In a state the target temperature is low is used as a reference, within a range the harmonic specifications are satisfied, if generation of a control pattern in which the number of times of the phase control in one control cycle is increased as much as possible is performed, when the target temperature is increased, due to the increase in the heater current, the harmonic specifications are not satisfied. On the other hand, in a state the target temperature is high is used as a reference, if generation of a control pattern in which the number of times of the phase control is small is performed, when the target temperature is lowered, the harmonic specifications are satisfied. However, the heat generation unevenness of the heater is large, and this is disadvantageous to image quality of print image.